FIELD OF THE INVENTION
The invention lies within the field of safety devices for nuclear power plants. The invention relates to a blow-off device for live steam from a nuclear power plant, in which the blow-off device is in communication with at least one live-steam line of the nuclear power plant. The invention also relates to a method of blowing off live steam from a live-steam line of a nuclear power plant.
In a nuclear power plant, steam generated in a reactor pressure vessel or in a steam generator is directed through a live-steam line to a turbine. The reactor pressure vessel and the steam generator are accommodated in a reactor building, which has a safety container surrounded by a protective casing. The turbine is located in a turbine building situated outside the reactor building. A line opening into the atmosphere branches off from the live-steam line upstream of a first stop fitting, as seen in the flow direction. Adjoining that line is a line section with a blow-off control valve and a blow-off stop valve, as well as a section with a safety valve, which is connected in parallel with those valves. The line sections that are connected in parallel open out in a common blow-out line. The blow-out line, possibly together with blow-out lines belonging to other steam generators of the nuclear power plant, leads into a muffler or sound absorber, which has an outlet opening to the atmosphere.
The live-steam line as well as a feedwater line pass through both the safety container and the protective casing of the reactor building. Barriers between a primary circuit leading through the reactor pressure vessel and the atmosphere are therefore valve cones of the safety valve and of the blow-off stop valve and, in the case of a pressurized water reactor, also heating tubes of the steam generator, which separate the primary circuit from a secondary circuit.
In the case of a pressurized water reactor, the steam generated in the steam generator is non-radioactive in normal operation. In the event of a serious incident, damage could occur to the heating tubes in the steam generator, even if that is highly unlikely. As a result, radioactive substances would pass out of the primary circuit of the pressurized water reactor into the live-steam line. In that case, the valve cones of the safety valve and of the blow-off stop valve, together with valve seats, would form a barrier between the primary circuit and the atmosphere.
Even though the technology of valve cones and valve seats has been and is being constantly improved, 100% tightness of the seats cannot be guaranteed, since the possibility of foreign bodies or contaminants being located on the valve seat cannot be ruled out, especially if blow-off has been effected beforehand through the affected valves under the conditions of a serious incident.